The applications of wireless communication techniques have increased substantially in the last two decades. This has led to both speech and data services being transmitted in widely differing frequency bands. Essentially, the 400, 800, 900, 1800 and 1900 MHz bands are available worldwide for mobile speech transmission. With the introduction of the UMTS Standard (Universal Mobile Telecommunication System), the frequency range has been extended to 2170 MHz. As an alternative to landline telephony—keyword WLL (Wireless Local Loop)—the frequency range between 3400 and 3600 MHz has been released in various European countries in recent years. Where the aim is to transmit high data rates, this can now be done without the use of wires using the WLAN frequencies (Wireless Local Area Network). The frequencies released for these applications are in the 2.4 and 5.5 GHz range.
In order to make it possible to supply areas within buildings, such as commercial premises, airports, train stations, underground garages and hotels, with all of these services efficiently, an entire forest of antennas would be necessary if the individual antennas were to operate exclusively in the relevant frequency bands. There is, therefore, a demand to minimize this forest of antennas as far as possible. The aim is accordingly to have an antenna which covers, as much as possible, the frequency range from 800 to 6000 MHz and is suitable for use within buildings (so-called “in-house areas”).
One form of broadband antenna that is particularly suitable by virtue of its simplicity is the monopole antenna. The history of these broadband monopole antennas has been described, inter alia, in the article by Xu Liang et al., “Low-Profile Broadband Omnidirectional Monopole Antenna”, Microwave and Optical Techn. Lett., Vol. 25, No. 2, April 2000, p. 135-138, and in the article by N. P. Agrawall et al., “Wide-Band Planar Monopole Antennas”, IEEE Trans. on Antennas and Propagation, Vol. 46, No. 2, February 1998, p. 294-295. The first article describes rotationally symmetrical monopoles, while the second article covers the characteristics of planar monopoles in the form of a round or elliptical disk. The planar structure in this case has the advantage that it can be produced considerably more easily, and thus at a lower cost.
Such broadband monopole antennas are known. For example, U.S. Pat. No. 4,370,660 discloses a broadband monopole antenna with a planar elliptical disk with the aim of achieving a standing wave ratio (SWR) of less than 1.5 in a frequency range between about 800 MHz and 4.5 GHz.
GB Publication No. 2,236,625 discloses a broadband monopole antenna whose antenna element is in the form of a micro-stripline with two rectangular conductor surfaces on opposite faces of a dielectric substrate. This antenna is intended to make it possible to achieve a bandwidth ratio of better than 1:5 (frequency range between 700 MHz and 4 GHz) for a voltage standing wave ratio (VSWR) of less than 2.5:1.
U.S. Statutory Invention Registration No. H2016 (filed Mar. 5, 1986 and published Apr. 2, 2002) discloses a broadband monopole antenna in the form of a “mono-blade antenna” in which a single antenna element in the form of a blade is arranged above a base surface. An antenna such as this is intended to allow operating frequencies up to 8 GHz with a VSWR of less than 1.2:1.
The Agrawall article mentioned above discloses that the antennas described therein can achieve a VSWR of less than 1.5 at a maximum of 3.75-11.5 GHz (see FIG. 1 of the Agrawall article). This corresponds to a bandwidth ratio of only 1:3.1. As described above, however, it is desirable to provide an antenna for the frequency range from 800 to 6000 MHz, which corresponds to a bandwidth ratio of 1:7.5. In this case a VSWR (Voltage Standing Wave Ratio) of <1.5 should be achieved in all cases for this bandwidth ratio.